In a magnetic recording tape cartridge having a magnetic recording tape and a pair of hubs for winding the recording tape therearound, each of the hubs must be made with high accuracy with respect to the out-of roundness and flatness of the cylindrical surface of the hub for assuring correct running of the recording tape without deviation of the recording tape in a vertical direction. However, despite the fact that the hubs are solid, conventionally each of the hubs is molded as one integrated body by one molding process, therefore the hub tends to be deformed with sink marks formed on the cylindrical surface due to thermal deformation during the molding process, thereby lowering the accuracy of the out-of roundness and flatness or vertically of the cylindrical surface of the hubs. In a case where a magnetic recording tape is wound on the deformed hubs, the magnetic recording tape is also badly deformed by the deformed cylindrical surface, thereby resulting in unsteady tape running which causes deviation of the recording track and undesired recording and reproducing characteristics.
It has been quite difficult to avoid the occurrence of distortion on the cylindrical surfaces of the hubs so far as each of the hubs is formed by one molding process.